1. Field of the Invention
This invention relates to an improvement in an HF alkylation process, and particularly to an improved method of recovering hydrofluoric acid and acid soluble oil from a mixture containing HF acid, used as a catalyst in the process, and acid soluble oil, an undesirable by-product of the process.
2. Description of the Prior Art
It is common practice in the petroleum industry to produce high octane motor fuel by alkylating isoparaffins with olefins in the presence of a catalyst, which preferably is hydrofluoric acid or hydrogen fluoride (HF). The effluent from the alkylation reactor is usually passed to a settling vessel wherein a hydrocarbon phase is separated from an HF acid phase. The hydrocarbon phase is fractionated to separate low boiling hydrocarbons from the alkylate product. The acid phase is cooled and recycled to the alkylation reactor for use in the alkylation process. However, it is necessary to purify a portion of the recycled acid phase in order to control a buildup of acid soluble oils (hereinafter referred to as ASO) and water in the system. Generally, this purification is accomplished by passing a portion of an HF-containing stream from the settler to an HF purification or rerun fractionation column, wherein HF is stripped from the ASO by means of a vaporous hydrocarbon, which is preferably isobutane. While the HF purification systems employed heretofore have generally been effective to produce an acid stream having the required purity, the recovered ASO stream from the purification unit usually has contained undesirable amounts of HF, resulting in the loss of valuable HF and in ecology problems. This has required subsequent purification steps to remove HF before thE ASO can be processed further, such as by recycling to the refinery as fuel, or passed to a disposal unit. Typical alkylation processes are shown in U.S. Pat. No. 2,574,006 issued to Allen, and U.S. Pat. No. 3,726,940 issued to Cannady et al.
An improved method of controlling acid soluble oil in the circulating HF acid stream used in an HF alkylation process is disclosed in U.S. Pat. No. 4,199,409, the teachings of which are incorporated herein by reference. In this patent, a conventional HF acid rerun column is used to strip a portion of the HF acid stream from circulating acid containing ASO. The acid soluble oil stream obtained as a bottoms fraction in the rerun column is cooled and allowed to separate into two phases. The lower phase, which preferably comprises 75 to 90% of the material, is removed as a bottoms fraction from the separator. The lower phase is relatively rich in HF, while the upper phase in the separator is almost HF acid free. The patentee recognized that the heavy acid soluble oil streams are difficult to separate in practice, and that to achieve complete phase separation would require prohibitively long separation times or a prohibitively large separator vessel.
Another method of recovering acid soluble oil is disclosed in U.S. Pat. No. 4,404,418, the teachings of which are incorporated herein by reference. In this patent, the HF acid containing ASO is heated, by any means suitable to generate heat, wherein the lower boiling HF is removed overhead as vapor, and the higher boiling ASO is separated out and remains in liquid form and is removed as product from the HF rerun column. This acid soluble oil is cooled and then diluted with liquid isobutane recovered from the upper liquid hydrocarbon phase in the settler, which was changed to a separation zone. The diluted acid soluble oil in isobutane is then charged to a separation zone, where any remaining HF catalyst is driven off. The recovered acid soluble oils dissolved in isobutane are then sent to the HF alkylation zone in an amount required to ensure production of a quality HF alkylation product.
Other attempts to separate ASO from a portion of the HF phase are disclosed in U.S. Pat. Nos. 4,373,110 and 4,454,369, the disclosures of which are herein incorporated by reference. In U.S. Pat. No. 4,373,110, at least a portion of the ASO is combined with at least a portion of the hydrocarbon phase for an alkylation unit. In U.S. Pat. No. 4,454,369, ASO is separated from HF acid catalyst in a fractionation zone by heating the fractionation zone with a condensable stream comprising liquid and vaporous components. In the process of condensation, latent heat is evolved which is used to effectuate the separation of ASO and HF acid catalyst, and dilution of the separated ASO is achieved with the use of the produced condensate.
The present applicants reviewed the work that others have done in cleaning up the circulating acid stream in an HF alkylation plant and recovering the acid soluble oil, but were not able to find any completely satisfactory solution. Acid-containing streams are extremely corrosive on the processing equipment, requiring specialized metal alloys and increasing the capital cost of the equipment. Simply neutralizing the HF acid containing in an acid soluble oil stream, obtained from the bottom of a conventional HF acid rerun column to minimize corrosion, represented a needless loss of the HF acid contained in this stream. The solution proposed in U.S. Pat. No. 4,199,409, while an improvement, required a difficult phase separation before the process could be successfully implemented.
Simply cooling the HF acid-soluble oil stream withdrawn from the bottom of an HF acid rerun column was not a good way to produce a two-phase mixture, because of the difficulty of separating the two phases which formed after cooling. The concentration gradient obtained by simple cooling was excellent. U.S. Pat. No. 4,199,409 reported that the HF-rich phase contained approximately 150 times as much acid, on a weight percent basis, as to the HF-lean phase. The problem was the difficult phase separation.
Cooling the acid soluble oil and the diluting with liquid isobutane, as proposed in U.S. Pat. No. 4,404,418, did not address the problem of corrosion of the equipment, since the diluted ASO was fed directly to subsequent processing equipment, such as an isostripper, without phase separation.
Settling for less concentration gradient, or differential in HF concentration between the HF-lean ASO and the HF-rich ASO, would be more than compensated by the ease in phase separation afforded by addition of a relatively light hydrocarbon stream or equivalent solvent to the acid soluble oil stream withdrawn from the bottoms of a conventional HF acid rerun column, while at the same time avoiding the corrosion problems present in prior art processes.